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Why Molds Need Sprues and Vents

Mold making and casting are relatively simple processes. Make a mold of the model you want to reproduce and then pour the casting material into the mold. Wait for it to cure properly before extracting, and viola your artwork is ready!

However, certain technicalities also creep into the picture. For instance, you have to choose the right mold making and casting products that are suitable for the job. At times, release agent, fillers and other items are also required.

Another important point is that as the casting material poured through the sprue, displaces the air in the mold, some air can easily get trapped inside and the casting material will be unable to completely fill the mold. These air pockets cause hollow spaces which can result in an unsightly and deformed cast.

So, how will you get rid of these air traps and pockets when casting silicone rubber or polyurethane resin?

The answer is simple – make vent holes! These are holes that are created in the sides of the mold usually on the high spots of the mold where air tends to first migrate. This allows air to escape from the mold when casting material is being poured inside. Now the casting material can reach all the corners and spots in the mold sans any gaps.

The location and number of vents is important. The holes are ideally placed along the top surface of the mold as air tends to rise to the top of the mold. It’s best to create vents all along the top perimeter of the mold while ensuring that you cover all the high spots. Keep them an inch or so apart. If you are making sprues on the sides of the mold, ensure that they taper up or else the casting material will easily flow out of the mold!

How to?

Vents are made by drilling holes in the surface of the finished mold. Alternatively, you can even insert hollow tubes during the mold making process itself. When removed later, the tunnel will serve as a sprue.

The same vent holes can serve a dual purpose. Apart from working as a vent for air to exit the mold, the hole can also be used as an alternate sprue, to add liquid casting material into the mold. Think injection molding!

In case the same vent is used as a sprue hole and vent, it should be located at the highest point of the mold and be large enough to allow the air to escape even while the material is being poured.

Carefully pour the silicone rubber or polyurethane resin into the sprue hole and continue till it overflows from all the vents. This shows that the material has displaced all the air inside. The material will cure in the vents too and can be cut be cut off after demolding.

Please not that only closed molds require vents. If the mold is open at the top (will become the flat bottom of the cast), the air can escape easily and the material will fill out without requiring any vents.

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